The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode during piloting. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, wherein the pilot arc heats and subsequently ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
In one mode of operation, commonly referred to as “piercing,” the plasma arc torch is started at a location on the workpiece rather than on an edge of the workpiece to start a cut. Piercing becomes more difficult as the workpiece thickness increases, and in general, piercing workpieces that are thicker than about one inch is often challenging. Additionally, piercing thinner workpieces at lower current levels can prove to be difficult as well. With thinner workpieces, the pierce time is relatively short and the arc has a tendency to stretch as material is removed rather quickly. The stretched arc can cause damage to components of the plasma arc torch, such as the tip, and can also cause an over voltage condition such that the power supply cannot deliver the requisite amount of power. Moreover, during piercing operations, molten metal, or slag, has a tendency to splatter onto components of the plasma arc torch and reduce their effectiveness and overall useful life. Therefore, significant efforts are undertaken to design proper gas shielding to protect the plasma arc torch and its components from molten slag during piercing.
During piercing, the plasma arc creates a semi-ellipsoid shape in the workpiece, and molten metal travels away from the pierce location, taking on multiple trajectories and spanning radially and azimuthally. In order to deflect the molten metal away from the plasma arc torch and its components, and also to cool the molten metal such that it has less of a tendency to adhere to components of the plasma arc torch, shield gases are employed to exert a proper deflection force and for cooling. Compared to controlling current, the type and amount of shield gas is often difficult to control in order to effect proper deflection/cooling of the molten metal, and thus improved methods of piercing are continuously being pursued in the art of plasma arc cutting.